Tighter fuel specifications coupled with more stringent environmental regulations have compressed refinery margins. There is a growing drive to cost-effectively maximize production of more valuable, lighter fuel products from heavy portions of every barrel of crude oil processed.
Crude oils are complex mixtures of hundreds of different species of chemical compounds. Higher value crude oils are typically referred to as lighter “sweet” crude oils while heavier crude oils are known as “sour”, as they contain high concentrations of sulphur (S), nitrogen (N) and oxygen (O) together with metal impurities such as vanadium (V) and nickel (Ni). The oil processing industry has an inevitably finite feedstock. Light feedstocks are steadily being replaced by the heavier crude oils or even alternative types of feeds altogether (such as bitumen derived from oil sands). Not only does the processing of these heavier feedstocks usually result in lower yields of the desired lighter products, but the higher concentrations of the various contaminants makes this processing more difficult and hence more expensive.
The specific gravity of crude oil and petroleum products is generally expressed in degrees API (American Petroleum Institute). API gravity is an inverse measure of a petroleum liquid's density relative to that of water (also known as specific gravity). An API of 10° is equivalent to water. It means that a petroleum liquid with an API greater than 10° will float on water while any with an API below 10° will sink. While API gravity is a dimensionless quantity, it is referred to as being in ‘degrees’. API gravity is gradated in degrees on a hydrometer instrument. If one petroleum liquid is less dense than another, it has a greater API gravity. API gravity values of most petroleum liquids fall between 10 and 70 degrees.
Therefore, heavy crude oils, having an API gravity of less than 20°, suggest high viscosity, a high content of polynuclear compounds and relatively low hydrogen content. Extensive Reserves of heavy crudes are found in a number of countries, including Western Canada, Venezuela, Russia, the US and elsewhere. Heavy crudes also include distillation residues, visbreaker tars, thermal tars, etc.
Crude oils need to be processed and refined into more useful products such as: gasoline, diesel, kerosene, etc. Most refineries, regardless of complexity, perform a few basic steps in the refining process, including but not limited to: distillation, cracking, treating and reforming. Distillation separates the hydrocarbons against boiling points. An atmospheric distillation unit separates the lighter hydrocarbons from the heavier oils based on boiling point. To increase the production of high-value petroleum products, these heavier oils left in the bottom of the distillation unit are run through a vacuum distillation column to further refine them.
The product that is left at the bottom of a vacuum distillation unit is referred to as a vacuum bottom, which is the heaviest material in the refinery tower. Fluid catalytic cracking (FCC) is primarily used in producing additional gasoline in the refining process. It is a chemical process that uses a catalyst to convert the high-boiling, high-molecular weight hydrocarbon fractions of petroleum crude oils to more valuable gasoline. Heavy cycled gas oil is the bottom product of FCC and is referred to as slurry oil that contains catalysts not captured by cyclones in the FCC unit.
Similar to heavy crudes, both slurry oils and vacuum bottoms are also considered as heavy fuels. Two primary routes exist for the conversion of such feeds, both serving to reduce the C:H ratio, hence resulting in a decline in the viscosity, boiling point and solid formation tendencies of the feed. These routes involve either reducing the amount of carbon or increasing the hydrogen, termed “carbon rejection” and “hydroconversion” respectively.
The carbon rejection process (also referred to as the coking process) is operated at elevated temperature and pressure; see Table 1 below for processing details which can vary significantly depending on the process being used. These processes include visbreaking, fluid coking or delayed coking, and flexicoking, which relies solely on thermally initiated radical reactions to both crack larger, higher boiling molecules into lighter species and to condense carbon-rich radical fragments into coke. The removal of carbon as coke results in an overall reduction in the C:H ratio for the liquid species, manifesting itself as a decline in the viscosity and average boiling point temperature. The low value coke by-product, which may be present in up to 20 wt % of the final product, is heavily contaminated and represents a significant environmental hazard. In addition, carbon rejection processes frequently produce incompatible two-phase products and de-asphalting results in a low yield of syncrudes.
TABLE 1ProcessProcess conditionsVisbreakingMild thermal cracking (low severity)Mild (470-500° C.) heating at 50-200 psigImprove the viscosity of fuel oilDelayedOperates in semi-batch modeCokingModerate (480-515° C.) heating at 90psig, Soak drums (450-480° C.° F.)Fluid CokingServer (510-520° C.) heating at 10 psigOil contact refractory coke Bedfluidized with steam-even heating,Higher yield of light ends (<C5), Lesscoke yieldFlexicokingA continuous fluidised bed technologywhich converts heavy residue to lightermore valuable product. The processessentially eliminates the cokeproduction. Temperature 510-540° C.
Hydroconversion operating conditions vary greatly, with temperatures ranging from 370 to 450° C. and pressures from 0.7 to 2.7 MPa, depending on the reactor type (typically fixed bed, fluidised bed or slurry-phase), catalyst type and feed. This process is often conducted in the presence of either a supported metal catalyst, such as NiMo/Al2O3, or an unsupported metal catalyst, such as Fe or Mo for example. Similar to the carbon rejection process, cracking within a hydroconversion reactor occurs by radical reactions initiated by the elevated temperatures, with coke being formed by condensation reactions between radicals. The catalyst can activate hydrogen dissolved in the residue oil to form free hydrogen radicals which then stabilise hydrocarbon radicals and hydrogenate the molecules, resulting in an overall decrease in the C:H ratio. Hydroconvensions normally generate high quality products but require high pressure of hydrogen gas and frequent regeneration of catalysts, leading to a high cost.